I've borked it

[siggi]

Hopefully you have plugged in the CPU and ULA the right direction, NOT as shown in the ZX81 manual chapter 25:

https://www.manualslib.com/manual/12017 ... =71#manual

[msknight]

Hopefully you have plugged in the CPU and ULA the right direction, NOT as shown in the ZX81 manual chapter 25:
https://www.manualslib.com/manual/12017 ... =71#manual

Good grief. Who on earth let that past quality control!

No, I've just got a problem in that I've been pulling chips in and out of sockets for testing combinations that the pins are getting about as weak as my will to continue.

[Dim_75]

This "tech-guy"' may need some of these

[msknight]

I'm trying to figure this out and getting nowhere.

For the HM65256B, looking at the read cycle, both the Chip Enable Access time and the Column Address Hold time, which both run from Chip Enable running low ... are both 150ns. With an "Output hold time from column address of 5ns... it's as if there's no time at all for the processor to actually read any valid data from RAM.

This is making no sense to me.

[1024MAK]

Reading the diagrams on a datasheet is an art

Keep in mind that the diagram itself can be misleading if taken literally. In the real world, the "width" of the various features on the diagram depends on a number of variables, including the clock/bus access speed of the system. Which in the ZX81 are more complex than normal Z80 systems due to the way that the ULA and the Z80 work to generate the data stream for the video.

Okay, the site that I am going to link to is for the 6502, but keep calm, and do have a look, as it does make the timing diagrams make a little more sense. Here's the link.

Back to the Z80 and 32k byte SRAM chips. Most of these all work the same. After all, they were (mostly) all being used for the same function.

Mark

[msknight]

That's a comprehensive document, but while I can understand what its saying, I'm having trouble identifying the mismatches because some of the chip diagrams are different enough that I can't then make the leap to identify what I need to do, to solve the problem... if that makes sense.

[Moggy]

Having exhausted all other repair possibilities a more drastic solution is found!

[msknight]

OK - after much scrumpled pieces of paper, I can't work it out.

Help! Please!

[1024MAK]

"These aren't the droids you're looking for".

If, and it's a big if, I can find a little time at the weekend, I want to try something which may or may not help.

Far too tired now. Dogs woke me at 04:50 just before my alarm was due to go off, as I had to catch an early train to take me to London. Now on the train on the way back.

I'm gonna be going to London next week as well...

Mark

[msknight]

It would be wonderful if you could... it's the last piece in the jigsaw for me now.

But I fully understand. Life is difficult enough without commuting to London on top.